According to recent studies, in motorized countries about half of all fatal accidents are single vehicle crashes. Studies also show that unintentional roadway departures accounts for the highest share of these traffic related fatalities.
The automotive industry has developed active safety systems that aim to prevent or mitigate accidents. One example is yaw stability control systems that assist the driver in regaining control of the vehicle. Yaw stability control systems have proven to be very efficient in reducing the amount of fatalities in traffic.
Conventional yaw stability control systems rely heavily on the driver's actions when controlling the motion of the vehicle. Based on the steering wheel angle provided by the driver, the yaw stability control system computes a desired trajectory which is tracked when the vehicle operates in an unsafe region of the state space, in order to maintain safe travel.
A measure based on the vehicle's yaw rate is used in conventional yaw stability control systems to identify when the driver has lost control and requires assistance. This measure may be seen as a comparison between the vehicle's actual trajectory and an interpretation of the trajectory that the driver intends to follow. If the difference between the driver's intentions and the vehicle's actual movement becomes too large the system decides to assist the driver in following the intended trajectory.
Interpretation of the driver's intentions is done by feeding the driver's input, i.e. steering angle through a simplified vehicle model with the assumption that it corresponds to the driver's perception of a vehicle's behavior. The simplified vehicle model that is used to compute the intended, or equivalently the reference trajectory in conventional yaw stability control systems is normally a single track vehicle model, according to FIG. 1. In the simplified model the lateral tire force at each tire Fyf, Fyr is approximated to be linearly related to the tyre slip angle, α. With this view, one may say that the conventional yaw stability control system aims at making the car follow the driver's intentions. The vehicle slip angle β is also illustrated in FIG. 1 and is defined as the angle of the velocity vector v in the vehicle's coordinate system x, y.
By commanding a desired trajectory, a skilled driver may efficiently use the yaw stability control system in challenging situations to keep the vehicle on the road. However, normal or inexperienced drivers of a vehicle equipped with a conventional yaw stability control system, may behave incorrectly in such challenging situations, due to e.g. panic, and fail to guide the yaw stability control system in order to maintain a safe trajectory. In fact, it is common that vehicle motion reaches the limit of adhesion between tire and road due to panic reactions of the driver.